In recent years, an MRAM that utilizes magnetoresistance effect is proposed as one of nonvolatile memories. In particular, an MRAM using an MTJ (Magnetic Tunnel Junction) that exhibits great magnetoresistance effect is actively developed.
A typical MTJ has a laminated structure in which a nonmagnetic insulating layer (hereinafter referred to as a “tunnel barrier layer”) is sandwiched between a first ferromagnetic layer and a second ferromagnetic layer. At a time when a current flows in a direction perpendicular to a film surface of the laminated structure, an electric resistance of the MTJ varies depending on a relative angle between respective magnetizations (magnetic moments) of the two ferromagnetic layers. More specifically, the electric resistance takes the minimum value in a state where the respective magnetizations are parallel to each other, while it takes the maximum value in a state where they are anti-parallel to each other. The variation in the resistance value is called “TMR (Tunneling Magneto Resistance) effect”.
In the MRAM, an element (TMR element, MTJ element) having such a MTJ is used as a memory cell, and the high and low resistance values of the MTJ are related to data “1” and “0”. Data read is achieved by detecting the resistance value of the MTJ. On the other hand, data write is achieved by switching the magnetization state of the two ferromagnetic layers between the “parallel state” and the “anti-parallel state”. In general, the magnetization direction of one of the two ferromagnetic layers is fixed, and the data write is achieved by reversing the magnetization of the other thereof. The former is called a “pinned layer (magnetization fixed layer)”, and the latter is called a “free layer (magnetization free layer)” or a “recording layer”.
Conventionally known methods of the data write to the MRAM include an “asteroid method” (refer, for example, to U.S. Pat. No. 5,640,343) and a “toggle method” (refer, for example, to U.S. Pat. No. 6,545,906 and Japanese Patent Publication JP-2005-505889A). According to these write methods, a magnetic switching field necessary for switching the magnetization of the free layer increases in substantially inverse proportion to a size of the memory cell. That is to say, a write current tends to increase with miniaturization of the memory cell.
As a write method capable of suppressing the increase in the write current with the miniaturization, there is proposed a “spin transfer method” (refer, for example, to Japanese Patent Publication JP-2005-93488A and J. C. Slonczewski, “Current-driven excitation of magnetic multilayers”, Journal of Magnetism and Magnetic Materials, 159, L1-L7, 1996). According to the spin transfer method, a spin-polarized current is injected to a ferromagnetic conductor, and direct interaction between spin of conduction electrons of the current and magnetic moment of the conductor causes the magnetization to be switched (hereinafter referred to as “spin transfer magnetization switching”). Here, it is known that a threshold value of the spin transfer magnetization switching depends on “current density”. Therefore, the write current necessary for the magnetization switching decreases with the reduction of the size of the memory cell.
In a case where the spin transfer method is applied to the MTJ, the write current is supplied between the pinned layer and the free layer through the tunnel barrier layer. The magnetization of the free layer can be reversed by the spin transfer (transfer of spin angular momentum) between the pinned layer and the free layer. In this case, however, the write current flows so as to penetrate through the laminated structure of the MTJ, which may deteriorate the tunnel barrier layer.
There is also proposed a method that supplies an in-plane write current (for example, Japanese Patent Publication JP-2005-191032A and Japanese Patent Publication JP-2006-073930A). The method will be described with reference to FIG. 1.
In FIG. 1, a magnetoresistance effect element has a magnetic recording layer 110, a pinned layer 112 and a tunnel barrier layer 111 sandwiched between the magnetic recording layer 110 and the pinned layer 112. The magnetic recording layer 110 has a first magnetization fixed region 110-1, a second magnetization fixed region 110-2 and a magnetization switching region 110-3. The magnetization switching region 110-3 overlaps with the pinned layer 112 to form an MTJ together with the tunnel barrier layer 111 and the pinned layer 112.
The first magnetization fixed region 110-1 is connected to a first boundary B1 of the magnetization switching region 110-3. On the other hand, the second magnetization fixed region 110-2 is connected to a second boundary B2 of the magnetization switching region 110-3. The magnetizations of the first magnetization fixed region 110-1 and the second magnetization fixed region 110-2 are fixed in the opposite directions. In FIG. 1 for example, the magnetization of the first magnetization fixed region 110-1 is fixed in the −X-direction, and the magnetization of the second magnetization fixed region 110-2 is fixed in the +X-direction.
Whereas, the magnetization of the magnetization switching region 110-3 is reversible and can be directed toward the first boundary B1 or the second boundary B2. As a result, a domain wall (domain wall) DW is formed at the first boundary B1 or the second boundary B2 in the magnetic recording layer 10. In FIG. 1, the magnetization direction of the magnetization switching region 110-3 is the +X-direction and thus the domain wall DW is formed at the first boundary B1. The data “1” or “0” is determined depending on the relationship between the magnetization direction of the magnetization switching region 110-3 and the magnetization direction of the pinned layer 112.
Also, as shown in FIG. 1, a first interconnection 131 is connected to the first magnetization fixed region 110-1 through a first contact 132, and a second interconnection 134 is connected to the second magnetization fixed region 110-2 through a second contact 133. In the data write operation, a write current is supplied between the first interconnection 131 and the second interconnection 134. That is, the write current flows within a plane in which the magnetic recording layer 110 is formed and thus does not penetrate through the tunnel barrier layer 111.
For example, in the state shown in FIG. 1, the write current flows from the second interconnection 134 to the first interconnection 131 through the magnetic recording layer 110. In this case, electrons are injected from the first magnetization fixed region 110-1 into the magnetization switching region 110-3 through the first boundary B1. Since the spin electrons of the first magnetization fixed region 110-1 are injected into the magnetization switching region 110-3, the magnetization of the magnetization switching region 110-3 is reversed in the −X-direction due to the spin transfer. As a result, the domain wall DW is formed at the second boundary B2. In other words, the domain wall DW moves from the first boundary B1 to the second boundary B2 through the magnetization switching region 110-3 due to the in-plane write current.
On the other hand, in a case where the write current flows from the first interconnection 131 to the second interconnection 134 through the magnetic recording layer 110, the domain wall DW moves from the second boundary B2 to the first boundary B1 through the magnetization switching region 110-3. As a result, the magnetization of the magnetization switching region 110-3 is reversed in the +X-direction.
In this manner, the domain wall DW in the magnetic recording layer 110 moves between the first boundary B1 and the second boundary B2 due to the current flowing between the first magnetization fixed region 110-1 and the second magnetization fixed region 110-2. This phenomenon is called “current-driven domain wall motion”. The data write method utilizing the current-driven domain wall motion is called a “domain wall motion method”. The domain wall motion type MRAM is characterized by suppressing the deterioration of the tunnel barrier layer 111. Moreover, since the data write is achieved based on the spin transfer method, the write current can be reduced with the reduction of the size of the memory cell.